A current challenge to cable manufacturers is to produce a cable that avoids “spikes” in Near End Crosstalk (NEXT) and Far end Crosstalk (FEXT) at transmission frequencies up to 2 Ghz. Crosstalk is the result of radiational coupling between twisted wire pairs situated in close proximity to each other. A situation that must be minimized in digital transmission cables. It is believed that repetitions in the cable lays (occurring naturally or resulting from manufacturing defects) cause coupling to add constructively, resulting in “spikes” in near end and/or farend crosstalk at certain frequencies.
In a current four pair cable using Unshielded Twisted Pair (UTP) or using Foil Shielded Twisted Pair (F/UTP) there are 6 combinations of possible twisted pair/twisted pair radiational interaction: (1) pair one to pair two, (2) pair one to pair three, (3) pair one to pair four, (4) pair two to pair three, (5) pair two to pair four) and (6) pair three to pair four. These combinations must be constructed with the spacing of repetitions or defects being outside the desired frequency range of the cable. The spacing of repetitions or defects must be greater than half the wavelength of the highest frequency of interest. Finding a suitable spacing of repetitions or defects is difficult when the frequency range is more than 500 Mhz because the shorter wavelength makes for fewer possible lay combinations that do not repeat in the given frequency range.
One solution is to shield all four twisted wire pair to eliminate coupling. The drawback of this solution is the increased size of the cable and the increased size of the twisted wire pairs themselves. In order to produce an individually shielded twisted wire pair with the required impedance, the insulation thickness of the wire must be significantly greater than that which is necessary for an unshielded twisted wire pair cable having the same impedance. This increases the overall cost, size, and stiffness of the cable.
Another solution is to increase the size or thickness of a separator or filler used to assure an appropriate distance is maintained between the twisted wire pairs. However, this method also increases the overall size and stiffness of the cable.
A further solution is for the manufacturer to intentionally vary the twisted wire pair lays during the cable construction. This method, however, complicates the manufacturing operation, making the setup more difficult and increasing the chance of errors during the setup and construction of the cable.
The applicant's proposed design requires only one or two twisted wire pair combinations because a radiation shield isolates the twisted wire pairs into two groups of two twisted wire pairs each. The applicant's unique method of applying the radiation shield eliminates from consideration four (or possibly five) of the theoretical radiationally significant twisted wire pair interactions. The only interactions required to consider are (1) and (6) from the list of possible combinations noted above. In other words, the combination options are reduced to: twisted wire pair one combined with twisted wire pair two and separately, twisted wire pair three combined with twisted wire pair four.
It is, furthermore, possible that the particular lay combination of twisted wire pair one to twisted wire pair two can be used in the construction of both groups of twisted wire pairs without this causing NEXT and FEXT spike issues. The applicant's unique method of applying the radiation shield also reduces the need to increase the insulation thickness in order to achieve the desired impedance because the shield is applied in a relatively loose manner around the twisted wire pair groups.